In inductive power supplies, an output voltage must be controlled such that it is suitable for loading an apparatus. In this control the output voltage is kept constant although a load changes. Normally the output voltage is controlled by a separate voltage regulator e.g. by a buck chopper (a buck voltage converter) or a boost chopper (a boost voltage converter) depending on an application. However, these kinds of inductive power supplies have low coupling or loose coupling capability meaning that a loading of a secondary side does not affect on the function of a primary side or does affect only slightly. Moreover, an efficiency ratio or a coefficient of a performance, and a total power or a total efficiency is usually on a relative low level in these power supplies with the low coupling. Thus, the use of separate voltage regulators reduces the total efficient coefficient of the apparatus, and the input power cannot be regulated in an efficient manner in relation to the requirements of the load.
In high coupling (or strongly coupled) inductive power supplies an input current depends strongly on the loading of the secondary side. In these power supplies Q-values of used resonators are high and power transfers to the secondary side with good efficiency ratio, although coils in the secondary side were away from each other. If a position of a secondary coil changes, also the coupling factor changes. This means that the efficiency ratio and the maximum efficiency to the secondary side changes leading to the fact that a feeding of dynamical loads in strongly coupled inductive power supplies is difficult. While it must be ensured that enough power can be transferred to the secondary side, the output voltage must be regulated by the separate voltage regulator similarly as with the case of low coupled power supplies.
The regulation of the output voltage with the separate voltage regulator does not only increase power losses, but also requires a lot of space from the circuit board of the secondary side. Moreover, the output power of the primary side cannot be controlled in an efficient manner with relation to the loading of the secondary side, and the efficiency ratio decreases remarkably.
Due to these facts the usage of separate voltage regulators is expensive and inefficient, because receivers need to produce high powers, while being small at the same time.
If data of the output voltage of the secondary side could be transferred to the primary side, it was not necessary to use the separate voltage regulator on the secondary side. In this case the input power could be controlled or regulated based on the requirement of the load increasing the efficiency of the power supply. In prior art solutions a separate radio link is used for this purpose, for implementing the voltage feedback in inductive power supplies. However, a voltage feedback coupling on a radio frequency has also its problems. Separate radio circuits must be implemented both in the primary side and in the secondary side. These additional circuits need space from the circuit board. Furthermore, if the primary side consists of many transmitting coils, the location of the receiver on a loading surface cannot be easily located on a radio connection.
The document U.S. Pat. No. 6,184,651 describes contactless battery charger with wireless control link. In it charging energy is transferred across an inductive coupler to charge a battery or a portable device. The system comprises an inductive coupler, a controller and a communication controller, a wireless RF receiver and a wireless RF transmitter, two secondary devices, and a sensing device.
The document U.S. Pat. No. 7,382,636 describes a system and method for powering a load. In it the power supply for inductively powering a remote device has an inverter operating on an operating frequency and a primary coil. A phase comparator compares the phase of the voltage or current delivered by the power supply. If the phase relationship detected by the comparator is unacceptable, the inverter is disabled. After a period of time, the inverter is re-enabled, and the phase relationship is again determined.